The invention relates to a circuit arrangement for distinguishing between the two fields in a television signal, comprising a detector adapted to assume a first or a second signal state in dependence on the time position of the synchronizing pulses contained in the television signal and a time measuring element connected to the detector for producing a field identification signal in dependence on the duration of one of the signal states.
Such a circuit arrangement is disclosed in German Patent Application No. 2,842,800. It generates a field identification pulse from the synchronising pulses with the aid of two series-arranged monostable trigger circuits having different time constants.
The synchronizing pulses of a television signal are usually formed from two pulse sequences which each characterize the start of a field and are located between a sequence of horizontal pulses. The two sequences comprise firstly a plurality of pre-equalizing pulses, then a plurality of field synchronising pulses and subsequently a plurality of post-equalizing pulses all the pulses occurring with a spacing half a line period. The spacing between the leading edge of the last horizontal pulse and the leading edge of the first pre-equalizing pulse and the spacing between the leading edge of the last post-equalizing pulse and the leading edge of the subsequent horizontal pulse is a whole line period for the first sequence and half a line period for the second sequence. In the European CCIR-standard five pre-equalizing pulses, picture synchronizing pulses and post-equalizing pulses are, for example produced.
In the prior art circuit arrangement a change in the output signal of the detector (the first monostable trigger circuit) occurs only then when the distance between the leading edges of two consecutive pulses exceeds a predetermined limit value. Here, this limit value must at least exceed half the line length. Consequently, a change in the output signal is only then produced after a horizontal pulse when the distance from the horizontal pulse to the subsequent pulse corresponds to one line period. During the field change this output signal is not changed, the duration of the unchanged signal depending on the field. The time measuring member (second monostable trigger circuit) produces a field identification signal after the longer duration fo the detector output signal. This field identification pulse appears after the first horizontal pulse after the last post-equalizing pulse.